As you already know, Python has a graphics package called tkinter that comes built-in with Python 3 (and Tkinter for Python 2!). While tkinter can be versatile in many ways, it can run slow and be lacking in features. A great alternative is Pygame -- it hosts a lot of features that make it useful for making not only games but for anything in general. It is, however, more complicated than tkinter, and anything you can write in Pygame can also be written in tkinter, so the choice is ultimately yours as to whether you want to use pygame or not.
Some of the contents in this book are heavily borrowed from Lukas Peraza's blog.